Fuel-cell-powered vehicles known today use hydrogen as their fuel; specifically, these fuel-cell-powered vehicles are provided with a traveling motor and a fuel cell stack that generates electric power using hydrogen and oxygen, so that they can travel by electricity. One example of a conventional method for operating a fuel cell assembly is known from Japanese Patent Laid-Open Publication No. SHO-54-22537, and one example of a conventional method for operating a fuel cell unit mounted on an automotive vehicle is known from Japanese Patent Laid-Open Publication No. 2000-514745 (WO98/04013).
FIG. 15 is a perspective view showing a fundamental construction and operation of the fuel cell assembly disclosed in the above-mentioned SHO-54-22537 publication. The disclosed fuel cell assembly includes fuel cell laminates 201, and bipolar plates 202 stacked alternately with the fuel cell laminates 201 and each having fuel gas channels 203 and oxygen-containing gas channels 204. With such arrangements, the fuel cell assembly generates electric power using the fuel gas and oxygen in the air, and the fuel cell laminates 201 can be cooled naturally with the air.
The fuel cell unit disclosed in the above-mentioned 2000-514745 publication includes a cooling system that cools fuel cells using traveling wind produced by traveling of an electric vehicle. The disclosed fuel cell unit further includes a pressure source (e.g., air blower or fan) for compulsorily passing air (traveling wind) through the cooling system. Traveling velocity of the vehicle is first detected to determine whether or not the vehicle is currently traveling at low speed, and then the fuel cells are cooled by the fan if the vehicle has been determined to be currently traveling at low speed. Alternatively, an external or ambient temperature is measured to determine whether or not the measured ambient temperature is relatively high, and then the fuel cells are cooled by the fan if the measured ambient temperature has been determined to be relatively high.
However, the fuel cell assembly disclosed in the SHO-54-22537 publication is not satisfactory in that heat produced in the fuel cell laminates 201 can not be dissipated or removed easily. Thus, there may arise a possibility of the temperature of the fuel cells getting excessively high, depending on environmental conditions, etc.
The fuel cell unit disclosed in the above-mentioned 2000-514745 publication is normally incorporated where a radiator of the vehicle is mounted, so that the cell unit (fuel cell stack) can be cooled directly by the traveling wind. In this case, however, there is a need for appropriately allocating the externally-introduced air for two purposes, cooling and electric-power generating purposes, in order to appropriately deal with an increase/decrease in the temperature of the fuel cell stack. As well known in the art, the fuel cells have an appropriate reacting temperature range for electric power generation; during a startup period of the fuel cell unit, for example, the fuel cells have a relatively low temperature and thus need not be cooled. However, in the disclosed fuel cell unit, the temperature of the fuel cells can not increase easily up to an appropriate level, during a startup period, because the fan is activated as long as the vehicle is in any of low-speed traveling states (including an idling state).